The attributes aren't available from the class that they aren't a part of:
(Pdb) ld = LocationDetail()
(Pdb) hasattr(ld, "starts_at")
False
(Pdb) ld.starts_at
*** AttributeError: Concrete mapped class LocationDetail->LocationDetail does
not implement attribute 'starts_at' at the instance level. Add this property
explicitly to mapped class LocationDetail->LocationDetail.
the fact that the attributes are there at all is an artifact of the way
AbstractConcreteBase works.
There is a way to exclude these properties from the base, which is to use the
include_properties collection:
class EventDetail(Base, AbstractConcreteBase):
uid = Column(Integer, primary_key=True)
@declared_attr
def event_id(cls):
return Column(Integer, ForeignKey(Event.uid))
@declared_attr
def event(cls):
return relationship(Event, back_populates="details")
__mapper_args__ = {
"include_properties": ["uid", "event_id", "type"]
}
However, this means that the querying the base EventDetail doesn't actually
*load* those properties; when you access them, an additional SQL query has to
be emitted. Even worse, they are actually in the original SELECT query but they
aren't assigned to the objects. I don't know why this is, as the more
well-maintained forms of inheritance (joined and single) don't have any of
these issues. These can all be considered to be bugs with concrete inheritance
but I don't have any solution for them right now.
(also make sure you set up back_populates on your mutually-dependent
relationships)
Since concrete inheritance is documented as being very buggy (see dragon at
https://docs.sqlalchemy.org/en/13/orm/inheritance.html#concrete-inheritance) I
would probably just make EventDetail into a non-mapped mixin class and then
have LocationDetail and SchedulingDetail load from separate relationships.
On Tue, Mar 3, 2020, at 3:14 PM, Isaac Martin wrote:
> SQLalchemy prescribes a pattern for dealing with polymorphic relationships
> which are one to many. You can see this pattern here:
> https://docs.sqlalchemy.org/en/13/orm/extensions/declarative/api.html#sqlalchemy.ext.declarative.AbstractConcreteBase
>
> After implementing this pattern I was pleased to find that it correctly
> created my tables for the child classes, did not create a table for the
> parent class, and successfully loads all children into a single collection on
> the one side of the one to many. Basically everything I wanted.
>
> Unfortunately, I quickly came to discover that all child classes had all the
> fields of all the other child classes. This doesn't happen on the tables, it
> is only present on the ORM objects after they are loaded. This presents an
> incredibly poor user experience for the developer. When they inspect these
> objects they discover a litany of irrelevant fields. Further, tools which
> visualize the data structure include these fields, making the visualization
> extremely confusing. For more in depth explanation of my specific
> implementation you can refer to the stack overflow post:
> https://stackoverflow.com/questions/60469773/sqlalchemy-polymorphic-pattern-creates-duplicated-columns
>
> Does anyone know how to prevent SQL alchemy from including fields from sister
> objects when implementing this pattern?
>
>
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
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